HBASE-19810 Fix findbugs and error-prone warnings in hbase-metrics (branch-2)

Signed-off-by: Michael Stack <stack@apache.org>

hbase-metrics/src/main/java/org/apache/hadoop/hbase/metrics/impl/HistogramImpl.java

@@ -65,6 +65,7 @@ public class HistogramImpl implements Histogram {
     histogram.add(value, 1);
   }
 
+  @Override
   public long getCount() {
     return counter.getCount();
   }
@@ -73,6 +74,7 @@ public class HistogramImpl implements Histogram {
     return this.histogram.getMax();
   }
 
+  @Override
   public Snapshot snapshot() {
     return histogram.snapshotAndReset();
   }

hbase-metrics/src/main/java/org/apache/hadoop/hbase/metrics/impl/MetricRegistriesImpl.java

@@ -51,22 +51,27 @@ public class MetricRegistriesImpl extends MetricRegistries {
     return registries.put(info, () -> factory.create(info));
   }
 
+  @Override
   public boolean remove(MetricRegistryInfo key) {
     return registries.remove(key) == null;
   }
 
+  @Override
   public Optional<MetricRegistry> get(MetricRegistryInfo info) {
     return Optional.ofNullable(registries.get(info));
   }
 
+  @Override
   public Collection<MetricRegistry> getMetricRegistries() {
     return Collections.unmodifiableCollection(registries.values());
   }
 
+  @Override
   public void clear() {
     registries.clear();
   }
 
+  @Override
   public Set<MetricRegistryInfo> getMetricRegistryInfos() {
     return Collections.unmodifiableSet(registries.keySet());
   }